1. Field of the Invention
The present invention generally relates to a satellite signal receivers and, more particularly, to mitigating interference in global positioning system (GPS) receivers.
2. Description of the Related Art
The use of portable personal communication devices, such as cellular telephones and pagers, has increased dramatically in recent years. Likewise, the use of portable navigational devices, such as global positioning system (GPS) receivers, has increased as such devices have become more widely available. Recent technological developments have allowed the integration of satellite signal receivers and communication systems within combined units to produce location-enabled mobile devices. For example, cellular telephones are now produced that include integrated GPS receivers.
GPS receivers use measurements from several satellites to compute position. GPS receivers normally determine their position by computing time delays between transmission and reception of signals transmitted from satellites and received by the receiver on or near the surface of the earth. The time delays multiplied by the speed of light provide the distance, or “pseudorange”, from the receiver to each of the satellites that are in view of the receiver. The GPS satellites transmit satellite-positioning data to the receivers, which is referred to as “ephemeris” data. The receiver uses the ephemeris data, along with absolute time information, to calculate where each satellite was when it transmitted a signal. Finally, the receiver combines the knowledge of satellite positions with the pseudoranges to the satellites to compute the receiver position.
More specifically, GPS receivers receive GPS signals transmitted from orbiting GPS satellites containing unique pseudo-random noise (PN) codes. Each satellite transmits a unique PN code (known as the C/A code) that identifies the particular satellite, and allows signals transmitted simultaneously from several satellites to be received simultaneously by a receiver with very little interference of any one signal by another. The GPS receivers determine the time delays between transmission and reception of the satellite signals by comparing time shifts between the received PN code signal sequence and internally generated PN signal sequences (“reference PN codes”). The comparison process between a received PN code and a reference PN code is referred to as a “correlation” process. The PN code sequence length is 1023 chips, corresponding to a 1 millisecond time period. One cycle of 1023 chips is called a PN frame. Each received GPS signal is constructed from the 1.023 MHz repetitive PN pattern of 1023 chips. At very low signal levels, the PN pattern may still be observed, to provide unambiguous time delay measurements, by processing, and essentially averaging, many PN frames. The averaging of multiple PN frames is referred to as “integration”.
To further enhance the reception characteristics, some GPS receivers may use “aiding” data that is transmitted to the mobile device. The aiding information is transmitted to the GPS receiver using some alternative form of communication (usually wireless, such as cellular data channels). By using aiding data, GPS receivers can operate in areas where signal levels are too low for traditional GPS to function properly.
One disadvantage inherent in many location-enabled mobile devices is the decreased performance of the satellite signal receiver section of the combined unit. A common cause for this decreased performance is signal interference between the wireless receiver section and the satellite signal receiver section. For example, in a combination cellular telephone/GPS receiver, cellular transmissions from the cellular telephone receiver section may generate interference that reduces the performance of the GPS receiver section. The satellite signal receiver section of the combined unit may also exhibit decreased performance due to signal interference between other wireless devices that are transmitting wireless signals in proximity to the combined unit.
One approach to overcoming cross-interference between wireless receiver and satellite signal receiver sections of a combined device involves disabling the “front end” of the satellite signal receiver when the wireless receiver is transmitting a signal. The front end is the portion of the satellite signal receiver that produces baseband signals from the received RF satellite signals for processing by a correlator. Another approach to overcoming cross-interference between wireless receiver and satellite signal receiver sections of a combined device involves disabling all satellite signal processing within the satellite signal receiver.
Disabling the front end of the satellite signal receiver, however, deleteriously affects the signal correlation process. Notably, the correlator will continue correlating even in the absence of baseband signals, which corrupts the correlation results with noise. In addition, in some cases, the wireless receiver section may be transmitting a low power signal such that the interference between the wireless receiver section and the satellite signal receiver section is minimal. In such cases, it is disadvantageous to disable the front end of the satellite signal receiver section, or all processing within the satellite signal receiver section, since signal correlation is unnecessarily suspended until the disabled circuitry is enabled.
Therefore, there exists a need in the art for a method and apparatus that mitigates interference in a satellite signal receiver without unnecessarily disabling circuitry within the satellite signal receiver.